A mass spectrometer is an analytical tool that may be used for qualitative and/or quantitative analysis of a sample. A mass spectrometer generally includes an ion source for generating ions from the sample, a mass analyzer for separating the ions based on their ratio of mass to charge, and an ion detector for detecting the separated ions. The mass spectrometer uses data from the ion detector to construct a mass spectrum that shows a relative abundance of each of the detected ions as a function of their ratio of mass to charge. By analyzing the mass spectrum generated by the mass spectrometer, a user may be able to identify substances in a sample, measure the relative or absolute amounts of known components present in the sample, and/or perform structural elucidation of unknown components.
Virtually all mass spectrometers include one or more multipole assemblies having a plurality of electrodes for use in guiding, trapping, and/or filtering ions. As an example, a multipole assembly may be a quadrupole having four rod electrodes, arranged as two opposing pairs. Opposite phases of radio-frequency (RF) voltage may be applied to the pairs of rod electrodes, thereby generating a quadrupolar electric field that guides or traps ions within a center region of the quadrupole. In quadrupole mass filters, a mass resolving direct current (DC) voltage is also applied to the pairs of rod electrodes, thereby superimposing a DC electric field on the quadrupolar electric field and causing a trajectory of some ions to become unstable and causing the ions to discharge against one of the rod electrodes. In such mass filters, only ions having a certain ratio of mass to charge will maintain a stable trajectory and traverse the length of the quadrupole, such that they are subsequently detected by the ion detector.
In multipole assemblies, the precision of the electric field (i.e., the degree to which the field approximates a desired, “pure” field) depends on the shape, position, and alignment of the electrodes. Electric field faults, which may arise from poor alignment of the electrodes or departures of the electrode shape and/or size from an ideal form, may cause excessive losses of ions when the multipole assembly is employed as an ion guide or ion trap, or poor resolution, sensitivity, and/or mass accuracy when the multipole assembly is utilized in a mass analyzer. Machining and aligning a multipole assembly with the small tolerances necessary to generate a highly precise electric field can be difficult and expensive, and conditions existing within a mass spectrometer can cause the relative positioning and alignment of the electrodes to change over time.